This userscript will replace the default diff collapse of GitLab by a lighter version that doesn't hog CPU. Tested with GitLab CE 12.7.6

gitlab-diff-collapse.js

// ==UserScript==
// @name         Gitlab diff collapse
// @version      1.0
// @description  Will "properly" collapse gitlab diffs by hidding the HTML instead of using Javascript magic that hogs CPU for a long time. Tested with GitLab CE 12.7.6
// @author       Romuald Brunet <[email protected]>
// @match        https://gitlab.com/*
// @match        https://-your-gitlab-instance-/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    const SEEN = 'seen-wgtrywW7L3SMXHjmP8IY';

    function diffToggle(event) {
        // May be another element
        if ( event.target.classList.contains(SEEN) || event.target.contains('diff-toggle-caret') ) {
            event.preventDefault();

            const target = this.parentNode.querySelector('.diff-content');
            if ( ! target ) {
                return;
            }
            if (target.style.display !== 'none') {
                target.style.display = 'none';
            } else {
                target.style.display = '';
            }
        }
    }

    function replaceElement(element) {
        // Replace the diff toggle element and its click event by our own
        if ( element.classList.contains(SEEN) ) {
            return;
        }

        observer.disconnect(); // avoid recursion

        // Remove previous listeners by cloning the element
        const clone = element.cloneNode(true);
        clone.classList.add(SEEN);
        element.parentNode.replaceChild(clone, element);

        clone.addEventListener('click', diffToggle);
        observe();
    }

    const observer = new MutationObserver(function(mutationsList, observer) {
        //console.time("Observe diff");
        mutationsList.forEach((record) => {
            record.addedNodes.forEach((node) => {
                if ( ! node.querySelectorAll ) {
                    return;
                }
                node.querySelectorAll('.diff-file > .file-title').forEach(replaceElement);
            });
        });

        //console.timeEnd("Observe diff");
    });

    function observe() {
        observer.observe(document.querySelector('body'),
                         {childList: true, subtree: true});
    }
    observe();

})();