My only experience with having a spinal tap was 20 years ago. It was uncomfortable but only took a few minutes so was easy to tolerate. I did have a major side effect which was a migraine from hell. This migraine lasted 3 days and was only relieved by another procedure at the hospital called a blood patch. Apparently there was a small leak of spinal fluid which caused horrible pain. I’ve heard that you must lay very still after a spinal tap for several hours to avoid this adverse reaction. The hospital got me up 20 minutes after the procedure.

I didn’t have much pain during the spinal tap.
They give you some mild anesthetic like lidocaine before they retrieve the spinal fluid.
The worst part is in the positioning, either you sit up and bend over something like a table, or you lie on your side.
Procedure itself lasted maybe 20 min
If they get in the space quickly, and they get the fluid quickly,the procedure is done.
After a spinal you do need to lie flat for awhile to prevent having a spinal headache.
I was lucky in that I was fine after the tap.
I had the spinal tap before the IVIG infusions even started..