For some reason text rendering seems to consume a lot of resources, such that a large amount of text on the screen tends to drastically drop frame rate in some scenarios.
Nameplates are an example of this when enough of them are shown, notably when a large amount of players get rendered on the screen or in servers that make use of armor stands as "holograms" to make floating text.

There are other cases where this issue is present as well, however as they're a lot i only listed the most notable ones:

• Entity nameplates / text_display entities (the most prevalent).
• HUD text (chat, tab list, sidebar, etc).
• GUI text (menus, item tooltips, etc).
• Glow signs (text rendered 9 times, see MC-226966).
• Obfuscated text ("scrambled" text effect, see MC-253502).
• Debug menu (F3).
• etc...

How to reproduce

1. Summon a few text display entities with the following command:

   /summon minecraft:text_display ~ ~ ~ {text:'ABCDEFGHIJKLMNOPQRSTUVWXYZ'}
   

2. Notice the frame rate, with enough nameplates you might notice the fps drop.