Installed the new patch, ran the intro cinematic again and what was barely watchable when I played it the first time was now pretty smooth. It stuttered only once during the quick fly by sequence and audio stayed on the entire time. A massive improvement from before.

It also seems that there might be campaign and level changes in the future patches. A dev on Steam forum said they have something cool planned soon.

EDIT:
It also messed up my save games. If you installed initially through offline installers but update using Galaxy to avoid having to download the entire thing again, a new user will be created by the patch. In the UserData folder, I have a folder called "0" which was the initial one and where the original saves are. After the patch, a new folder appeared and the game now takes saves from that one. I tried moving the old save files to the new folder, but they can't be loaded. Really sucks. Haven't found a way to make it accept the save games.

EDIT 2:
Finally managed to get the saves working again and surprise - it's Galaxy that messes things up. There are a few scenarios.

1) Install from offline installers without Galaxy running - creates a folder in UserData called "0" that it uses for saves.
2) Install from Galaxy - creates a folder in UserData called random numbers that it uses for saves.
3) This is the important one: Install from offline installers WITH Galaxy open or even running in the background (being visible in the bottom right bar) - it behaves the same as in situation 2.

The moment Galaxy touches the game, it ruins your save compatibility if you installed from offline installers.

Only when I reinstalled the game without Galaxy running (as I did originally) did the game finally accept my old saves. So just a word of caution. Guess I'm gonna have to re-download the entire game for every patch because Galaxy update ruins my saves. Gonna try to download the offline installers again tomorrow (hopefully with the new update) and see if I can transfer the saves that way at least. Even though I finished the game already, I would like to keep the history, stats etc.
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Damn. Turns out the stats and unlocked levels are gone anyway because I had to reinstall. I can load the saves but that's about it. Guess I'm just gonna install through Galaxy and be done with it. RIP my first playthrough...

Still, if this happens to you, you can use this as a temporary workaround to at least finish the game with your old saves.

This patch was indeed is very helpful: yesterday I installed the game, however when I tried to run the game right after the map would load there were graphic artifacts, unacceptable FPS (somewhere around 30) and then would crash in a few minutes. Tried to set lowest settings and run benchmark it would show about 30-40 FPS and then crash if benchmark is more then a minute... I could've understand if I was running it on some that barely meets minimum requirements, however GTX 1080 (with the drivers from a week ago), Ryzen 1700X & Win 10 is along the lines of the recommended config. Was thinking about giving it a few days and if it would be still broken refunding this, however after this 700Mb patch the game runs stable on high at a reasonable FPS (benchmark claims it is about 70FPS, which is probably true since I do not see any glitches or freezes and game seems to be running smooth).

Patch 1.01 55?

1.2 came out yesterday.

My version still says 1.01

Offline installers are still at 1.01. But the update is on Galaxy since yesterday.

I do not see any update version in that section

Didn't help for me. I still have bad performance.
Radeon RX 5500 XT
AMD Ryzen 7 3800 8 core
16g ram

Other than getting a better GPU, you can try tweaking the settings or try this:
https://www.gog.com/forum/serious_sam_series/big_performance_increase_for_ss_4_by_tweaking_an_ini_file

Worked wonders for my 1080ti, but not sure how much it will do for weaker cards.