plugin Complete_from_text is slow on 90K-lines text file

[Alexey-T]

text file:
bug5473_2x.zip

for Complete_from_text, open its config, and write:

plugins.ini:

[complete_from_text]
lexers=*
max_lines=100000

• open text file.
• go to end. type "a" and call auto-completion
  --> pause 5-6 seconds!
  VSCode is fast here.

@pintassilgo

with fix #5605 (comment) the pause is 2 secs, but still slower than VSCode.