Zyte Smart Proxy Manager(formerly Crawlera) is a proxy rotater which helps in evading bans from the target website by adjusting delays, handling cookies and managing IPs. When Smart Proxy Manager is added it can lead to slow down of jobs as it will adjust the delays to ensure crawl is not too fast, not overburden target server (thereby increasing the risk of recognition and bans).
But with Smart Proxy Manager we can control Concurrency that is Number of requests that can be made in parallel. This concurrency is based on the subscribed Smart Proxy Manager Plan. To increase the Crawl speed you can increase the concurrency. Concurrency is cumulative of requests made to all domains from all Smart Proxy Manager accounts in the Organization.
For Scrapy Cloud, Autothrottle is enabled by default which limits maximum number of concurrent requests sent to the same host domain. To increase the concurrency you would need to disable Autothrottle and set Concurrency settings in settings.py through UI as given in this article on Customizing Scrapy settings in Scrapy Cloud.
Please note each Smart Proxy Manager plan has limits on Concurrency and exceeding it would lead to errors with code 429.