An industrial B2B client was losing half of mobile traffic. WordPress with 24 plugins choked conversion and rankings. After a move to Next.js 16 + Sanity: LCP 3.8 s → 0.7 s, quote requests +140% in 60 days, hosting from 900 PLN/mo to 0. Full migration case study — SEO kept.
Starting point — technical debt
- LCP: 3.8 s (5.2 s on mobile) — Core Web Vitals red.
- INP: 380 ms.
- CLS: 0.24.
- Mobile bounce: 68%.
Migration plan — zero SEO loss
- URL audit — 340 pages mapped 1:1.
- Content import into Sanity.
- 301s in Next.js
middleware.ts, Search Console checks. - Parallel staging, 10% traffic A/B for a week.
- DNS cutover + 72 h monitoring.
Results after 60 days
| Metric | Before | After |
|---|---|---|
| LCP | 3.8 s | 0.7 s |
| Quote requests | baseline | +140% |
| TOP 3 keywords | 6 | 17 |
SEO survived because of the 301 map, canonicals and Search Console. Payback landed in ~45 days on +25 extra leads per month. Related: INP in 2026.
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