I concur that the number of mobs spawning in the Nether is insane. I have a roadway linking several of my overworld locations via the Nether. One leg is about 1000 blocks long in the Nether. I normally would encounter 3 - 4 Ghasts, which cause me to stop and engage them when running from one portal to the other. However, after the Nether upgrade I literally killed 20 of them and avoided another 20. I'd frequently stop to fight one or two and before I could dispatch them another 3 - 4 would spawn. I can not believe this is by design as it will make building or doing anything virtually impossible. I also encountered a very large number of Magma Cubes, as mentioned in comments above.
I would also like to note, one of my portals is in a village inside of a building. Villagers do have access to this building, and occasionally one or two would find their way into the Nether via the portal there. However, when I returned to this poral in the Nether, I found the area swarming with Villagers. There were literally 20 - 30 of them wandering around.
The excessive spawning my not be confined to the Nether, as when I returned to my village complex there seemed to be a very large number of Villagers in the overworld there, too. But, I have not noticed an unusual number of hostile mobs in the overworld, yet.
I have not created a new world or done other experimentation, as I have one primary world (about 2 GB) that I normally pay in.
I am hopeful the developers will also agree the above Nether mob spawning rate is excessive and tone it back to the previous levels. I think most experienced players will find the current level excessive.
This problem continues on 1.14.20. I have no lack of storage or memory. My world is about 1.8 GB. It can be reproduced at will. Moving several thousand blocks in one direction will trigger it every time. This can be in explored or unexplored territory. Find a nice long stretch of ocean, desert or plains and just start running, riding on a boat, horse, etc. The crash will occur. I have to shut the game down and restart it. I can not simply reload the world and continue. It just hangs if I attempt this. I have not lost items I am holding, or carrying. However, if I am riding a horse, boat, etc. these are often not there when the game reloads. Usually, it am a few seconds back in game play from where I was before the crash. The workaround is to periodically pause and save the game. The game has caused me to die several times, as it respawned me over the edge of a cliff I was near before the crash, etc. I suspect this has not risen to a high priority, as it will not occur as long as you stay in a confined area, or at least it hasn't for me. I have several houses which are a distance apart connected with overworld and nether railways. So, I used minecarts to travel between them. The carts are lost with the crash, as a note. I am playing Bedrock Edition. We also have another computer with Java Edition which does not seems to suffer this problem.
I am also experiencing this problem regularly. My current world is 1.5 GB. The easiest way for me to reproduce it is to travel very far by rail. I have several very long underground rail lines connecting far apart outposts. If I simply launch and wait after about minute the crash will occur. I say crash because after the warning that it was unable to save, the system will freeze and crash to desktop. The following error appears in the Application Event Viewer:
The program Minecraft.Windows.exe version 1.13.0.34 stopped interacting with Windows and was closed. To see if more information about the problem is available, check the problem history in the Security and Maintenance control panel.
Process ID: 389c
Start Time: 01d59843cf93768d
Termination Time: 4294967295
Application Path: C:\Program Files\WindowsApps\Microsoft.MinecraftUWP_1.13.34.0_x64__8wekyb3d8bbwe\Minecraft.Windows.exe
Fault bucket 2098508588997373242, type 5
Event Name: MoAppHang
Response: Not available
Cab Id: 0
Problem signature:
P1: Microsoft.MinecraftUWP_1.13.34.0_x64__8wekyb3d8bbwe
P2: praid:App
P3: 1.13.0.34
P4: 5da81653
P5: 628a
P6: 2097152
P7:
P8:
P9:
P10:
Attached files:
\\?\C:\ProgramData\Microsoft\Windows\WER\Temp\WERC68C.tmp.WERInternalMetadata.xml
\\?\C:\ProgramData\Microsoft\Windows\WER\Temp\WERC6BC.tmp.xml
\\?\C:\ProgramData\Microsoft\Windows\WER\Temp\WERC6CB.tmp.csv
\\?\C:\ProgramData\Microsoft\Windows\WER\Temp\WERC6FB.tmp.txt
These files may be available here:
\\?\C:\ProgramData\Microsoft\Windows\WER\ReportArchive\AppHang_Microsoft.Minecr_22c5fc3f85c5c411d19447411c66514f6618c3_da7994f8_986203a4-2744-45b9-9d87-58cea801c9b9
Analysis symbol:
Rechecking for solution: 0
Report Id: 910a4f00-1d4f-48c2-a557-329f655c82e8
Report Status: 268435456
Hashed bucket: 3eff5e24af14fe268d1f66717d4b6d3a
Cab Guid: 0
I have been exploring more widely in the Nether and have found that there are many more Magma Cubes than previously, as discussed and reported above. Also, frequently striking them with a sword they break into more than the usual four smaller cubes. Instead they break into the smallest cubes, 16 of them, with one strike. Suddenly, you find yourself swarmed. This has happened twice so far.
I mentioned above that I am playing on an old map that I have been developing for some years. I do not know how the dynamics of the mob creation works within the game code, and whether it is, in any way, related to the saved status of the map. In short, I wonder if this problem would appear if I generated a brand new map. I tested this using creative mode. I created a new map, went into the Nether and explored. I did not see an unusual number of mobs. But, this is creative not survival. Anyway, for someone trying to duplicate this issue, trying it on both an old and new map might be a good test unless there is no difference from the coding side. I did mention above that my villagers from the overworld spilled over into the Nether, as well. There were always a few that wandered through the portal. But, now there are dozens all around the Nether Portal. This is also not normal.