After importing 10,000 pages, the site began to slowly load

10k pages is possible but you need the setup to support it. I was curious and here's how I got it functioning.

Hardware

Google Compute Engine n1-standard-2 machine type VPS: 2 vCPU & 7.5GB RAM

• vCPU = hardware hyperthread on 2.2GHz Broadwell platform

Server software

• FreeBSD 11
• NGINX 1.12.1
• PHP-FPM 7.1.10
• APCu 5.1.8
• YAML parser (unknown if this has any impact)

php.ini adjustments

• memory_limit=1024M <- modify existing
• apc.shm_size=512M <- likely need to append - 512M is probably overkill

Grav

• Default Blog Site skeleton with plugins enabled
• Admin plugin installed
  • Note I also used the Precache plugin for testing purposes but did not impact results and ultimately it's not required
• 10,000 blog posts with 3 paragraphs lorem ipsum (1 visible as summary)

Results

• ~2.5s page load time (~2s TTFB)
  • Using Random or SimpleSearch takes a bit longer
• ~100MB APCu usage

Admittedly this isn't as 'snappy' as some other Grav sites I've built, but it has an acceptable level of functionality. I didn't try increasing hardware specs to improve performance but I'm sure additional gains can be made. E.g. I noticed a significant performance increase when I upgraded from 1 vCPU to 2, as php-fpm process is the bottleneck.

Now, I'm curious what your specific use-case is and how you define 'very long time'? Have you compared Grav's performance with 10k pages against another flat-file CMS?