What is the purpose of the PCIe X1 slot

Adapter placement rules and slot priorities for the EMX0 PCIe Gen3 I pcie expansion slot My SSD connected to my board but speedtest gives me 3Gbs If I buy pcie expansion card can it be faster than onboard slot? Onboard m2 slot or pcie slot length Full line of single and multi-slot Thunderbolt PCIe Card Expansion Systems to add PCIe cards to Mac and Windows computers

pcie 20 x1 slot IPCIE16XD11 is Multiple SFF-8654 x4 to PCIe x16 Slot Expansion with Broadcom MegaRAID 9560-16i, 9560-8i, 9580-8i8e, 9500-16i, 9500-8i,