You can summon in zombies at a specific location with a specific timer with command blocks:

Giving yourself a repeating command block or a chain command block:
/give @s minecraft:repeating_command_block
/give @s minecraft:chain_command_block

First make a scoreboard called timer:
/scoreboard objectives add timer dummy

Use a repeating commmand block that counts up the timer with this command:
/execute unless score 1/20sec timer matches 300 run scoreboard players add 1/20sec timer 1

Use a chain command block that resets the timer:
/execute if score 1/20 timer matches 300 run scoreboard players set 1/20sec timer 0

Summoning a zombie on a specific location every 15 seconds with a repeating command block (always active):
/execute if score 1/20sec timer matches 299 positioned x y z run summon zombie ~ ~ ~ {PersistenceRequired:1,active_effects:[{id:speed,amplifier:0,duration:-1,show_particles:0b}]}

x y z are the coordinates for your zombie location

If you want to give the zombie effects or not make any noise:
www.digminecraft.com/generators/summon_mob.php

Hope this helps :) Unfortunately zombies can't find their targets without seeing them first I made a lazy fix for this with on a repeating command block:
/execute as @e[type=zombie] at @s if entity @p[distance=10..] run tp @s ^ ^ ^1 facing entity @p[distance=10..] feet

This will make them teleport through walls though

So you can also do this but they won't be able to find you through walls:
/execute as @e[type=zombie] at @s facing entity @p[distance=10..] feet if block ^ ^ ^1 minecraft:air run tp @s ^ ^ ^1 facing entity @p[distance=10..] feet

Have fun!

meermeer3