The reason is if your website has WPBakery and Elementor installed, your website will load extra CSS and Java Scripts from each page builder, it will increased the files size and slow down website.
Another nightmare is plugin conflict. So I strongly WOULD NOT suggest using 2 page builder in the same website.
Size of WPBakery website
Take my favorite ThemeForest theme - Bridge as example. The page size of empty website with WPBakery is 1.04MB and 128 requests.
When I import a basic Business demo, the page size increase to 2.06mb with 158 request, fully loaded time increase 1.4sec to 3.3sec
Size of WPBakery and Elementor website
Let's try edit WPBakery website with Elementor with one headline.
With the additional of a SINGLE headline from Elementor, the page size increase 29.5KB and increase 13 extra requests, fully loaded time from 3.3sec to 4.8sec, additional of 1.4sec. You can see the impact is crazy.
Based on my test site, it have plugin conflict issue and breaks the website layout, the slider on the website no longer working.
Please note that these example is using RunCloud' NGINX stack with UpCloud, if you use shared hosting, the fully loaded time will even SLOWER.
The Best advice: Convert / Migrate WPBakery to Elementor