After installing package php7.4-mbstring, the speed of Import on php7.4-fpm & nginx 1.18.0-0 became identical to the speed of php7.4.3-4 & apache2.4.41-4.
Installing php7.4-mbstring does not affect performance with php7.4.3-4 & apache2.4.41-4.
Apparently php7.4.3-4 & apache2.4.41-4 can work with long strings by default without degrading performance.
And for php7.4-fpm & nginx 1.18.0-0 you need to install php7.4-mbstring for fast work with long strings.
Thanks again for the module