To mitigate the performance impact, optimize for the default case (`XFFDepth` is 0) by reverting to the more performant logic of using `strings.IndexByte` to find the first comma and then slicing the string. This avoids the overhead of `strings.Split` for the most common configuration and prevents a performance regression. The `strings.Split` logic should only be used when `XFFDepth > 0`.

**Example:**
```go
// ...
depth := 0
if Global != nil {
    depth = Global().HttpServerOptions.XFFDepth
}

if fw := r.Header.Get(header.XForwardFor); fw != "" {
    var ip string
    if depth == 0 {
        // Optimized path for default case
        if i := strings.IndexByte(fw, ','); i >= 0 {
            ip = fw[:i]
        } else {
            ip = fw
        }
    } else {
        // Path for new functionality
        xffs := strings.Split(fw, ",")
        if depth > len(xffs) {
            return ""
        }
        ip = xffs[len(xffs)-depth]
    }

    ip = strings.TrimSpace(ip)
    if parsedIP := net.ParseIP(ip); parsedIP != nil {
        return parsedIP.String()
    }
    // If IP is invalid, fall through to use RemoteAddr
}
// ...
```