Following rule MAY improve PHPStan run time. It makes sense only when you disable GC for PHPStan execution.

GarbageCollectorRule.php

<?php declare(strict_types = 1);

namespace ShipMonk\Rules\PHPStan\Rule;

use PhpParser\Node;
use PHPStan\Analyser\Scope;
use PHPStan\Node\InClassNode;
use PHPStan\Rules\IdentifierRuleError;
use PHPStan\Rules\Rule;
use function gc_collect_cycles;

/**
 * @implements Rule<InClassNode>
 */
class GarbageCollectorRule implements Rule
{

    private const int FREE_MEMORY_AFTER_CLASSES_ANALYSED = 100;

    private int $classCounter = 0;

    public function getNodeType(): string
    {
        return InClassNode::class;
    }

    /**
     * @param InClassNode $node
     * @return list<IdentifierRuleError>
     */
    public function processNode(
        Node $node,
        Scope $scope,
    ): array
    {
        if ($this->classCounter > self::FREE_MEMORY_AFTER_CLASSES_ANALYSED) {
            gc_collect_cycles();
            $this->classCounter = 0;
        }
        $this->classCounter++;

        return [];
    }

}

Disable GC e.g. by creating bin/phpstan and running that instead of vendor/bin/phpstan:

#!/usr/bin/env php
<?php declare(strict_types = 1);

gc_disable();

Phar::loadPhar(__DIR__ . '/../vendor/phpstan/phpstan/phpstan.phar', 'phpstan.phar');

require 'phar://phpstan.phar/bin/phpstan';