performance regression with big log

Today i tried to use pantheon terminal as primary (its ability to open dir in default file manager is amazing). I noticed, that when log growes, it begins to work slower and slower.

For test i used this command: `$ time base64 $(which alacritty)`
where...
```
$ base64 `which alacritty` | wc -l
75155
```
Comparing with st and alacritty and xfce4-terminal, first two do it nearly in 0.4s, xfce4-terminal - in ≈1.27s, without regression.
Below are results of above time command in pantheon terminal, invoked several times after terminal reset.

real 0m2.879s
user 0m0.031s
sys 0m0.170s

real 0m7.013s
user 0m0.032s
sys 0m0.155s

real 0m10.240s
user 0m0.031s
sys 0m0.163s

real 0m11.058s
user 0m0.038s
sys 0m0.149s

At this point terminal FPS was visually between 3 and 2.
Even just pressing enter (or echo something) showes result with roughly 0.4s.

cpu/gpu - intel b950 (sandybridge)