TL;DR
Thanks for all the input but I cannot reliably point to something and say, “look that became slower”. I typed on 0.7.6, 0.8.0, and 0.8.1 on the same day, and the input seems very close to each other.
Because there are performance improvements planned for 0.8.3, and since no one else mentioned typing lag, I think it’s a good idea to move this topic to ‘Resolved help’. 
Now for my lengthier response. 
On 0.8.0 I get the same CPU spikes and memory “mountains and valleys”:
The same thing also happens with 0.7.6:
So this behaviour, even though it looks a bit odd, cannot be attributed to a sense that input lag increased.
Because I was curious I also experimented to see what caused those CPU spikes and memory increases/decreases.
So I set out to experiment with each plugin disabled (successively) to find the problem:
| Plugin name |
Does disabling plugin solve CPU/memory pattern |
| Random note |
No |
| Daily notes |
No |
| Command palette |
No |
| Custom CSS |
No |
| Starred |
No |
| Tag pane |
No |
| Backlinks |
No |
| Graph view |
No |
| Quick switcher |
No |
| Search |
No |
| File explorer |
No |
This is the process that I followed:
- Disable plugin.
- Close Obsidian.
- Wait 20 seconds for proper closing.
- Start Obsidian.
- Wait 20 seconds for proper start up.
- Type plain text (3 paragraphs) in the editor and monitor utilisation of computer resources.
At the end of my test, I used Obsidian with all plugins disabled (and the left sidebar was gone, as Licat suggested). Because the CPU/memory pattern still happened then, it seems it doesn’t happen because of a plugin.
I also tried the typometer that was mentioned earlier in an attempt to measure the input lag of 0.7.6, 0.8.0, and 0.8.1, but I ran into a known issue. So unfortunately this objective way of measuring input lag didn’t work on my pc. 
(I also searched for other software to measure input lag but didn’t find any, sadly.)